UC San Diego - 26 April 2010
NASA's LRO Team Helps Track Laser Signals to Russian Rover MirrorA team of physicists led by a professor at UC San Diego has pinpointed the location of a long lost light reflector left on the lunar surface by the Soviet Union nearly 40 years ago that many scientists had unsuccessfully searched for and never expected would be found.
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Three reflectors are required to lock down the orientation of the moon. A fourth adds information about tidal distortion of the moon, and a fifth enhances that information.
NASA LRO Mission Pages - 26 April 2010
APOLLO program pinpoints location of Lunokhod 1 retroreflectorUsing information provided by NASA’s Lunar Reconnaissance Orbiter (LRO) instrument teams, researchers at the University of California San Diego successfully pinpointed the location of a long lost light reflector on the lunar surface by bouncing laser signals from Earth to the Russian Lunokhod 1 retroreflector.
The initial imaging of the two Russian rovers, Lunokhod 1 and 2 were made earlier this year by the Lunar Reconnaissance Orbiter Camera (LROC) team, led by Mark Robinson from Arizona State University in Tempe.
On April 22, Tom Murphy from the University of California San Diego and his team sent pulses of laser light from the 3.5 meter telescope at the Apache Point Observatory in New Mexico, zeroing in on the target coordinates provided by the LROC images and altitudes provided by the Lunar Orbiter Laser Altimeter
The Planetary Society Blog - 26 April 2010
With the recent Lunar Reconnaissance Orbiter imaging of the Lunokhod 1 rover, scientists on the APOLLO project were finally able to do something that scientists have been dreaming of for more than three decades: shoot the rover with a laser.
Wait, what?
First of all, I'm not speaking of the Apollo program that sent American astronauts to the surface of the Moon. I'm speaking of the Apache Point Observatory Lunar Laser-ranging Operation, or APOLLO. APOLLO is just one of several lunar laser ranging experiments that seek to measure the Moon's position in space with incredible accuracy by firing lasers at specially designed retroreflectors left on the lunar surface by past missions, and precisely timing how long it takes the reflected pulse to return to Earth. APOLLO is capable of measuring the Moon's distance from Earth to a precision of one millimeter.
That's cool, but why do it? There are several goals, but the overarching one is to test Einstein's Theory of Relativity, or, more specifically, the Equivalence Principle. This is not one of my areas of expertise so I'll just quote to you what the APOLLO website says in explanation:Einstein's Equivalence Principle, upon which General Relativity rests, claims that all forms of mass-energy experience the same acceleration in response to an external gravitational force. This is to say that the inertial mass and gravitational mass are equivalent for any form of mass and/or energy. This is very difficult to verify for gravitational energy itself, because laboratory masses have no appreciable gravitational binding energy. One needs bodies as large as the earth to have any measurable self-energy content. Even then, the self-energy contribution to Earth's total mass-energy is less than one part-per-billion.
If the earth's gravitational self-energy does not precisely obey the Equivalence Principle, the orbits of the earth and moon around the sun would be slightly displaced from one another (think of this as a modification of Kepler's third law), which would show up as a signal in our lunar range data. Various string-motivated theories, quintessence, and other alternatives to General Relativity almost all predict a violation of the Equivalence Principle at some subtle level. Given the recent hints that there may be some new and mysterious modification to the laws of large-scale gravitational attraction (as indicated by supernovae and cosmic background anisotropies), it is important that we probe every available aspect of the basic nature of gravity.
LLR also provides the best test of the constancy of Newton's gravitational constant, G, currently limited to a variation of less than a part in 1012 per year. Relativistic geodetic precession is also best probed (currently) by LLR, now verified at the 0.35% level of precision. And the list goes on: LLR provides the best test of the motional influence on gravitational attraction (called gravitomagnetism) at the 0.1% level, and also sets the most stringent limits on deviations from the expected 1/r2 law of gravity.